Inhibition of methane oxidation by ammonium in the surface layer of a littoral sediment

The diffusive flux of CH4 from the sediment into the water was measured in intact sediment cores taken from the littoral of Lake Constance. CH4 fluxes were measured under oxic and anoxic conditions. Emissions under anoxic conditions were used to estimate the potential diffusive flux. The results showed that about 79% of the CH4 diffusing upwards was oxidized at the sediment surface. Kinetic measurements of CH4 oxidation in diluted surface sediment gave Vmax = 1.2 μmol CH4 mlsed−1h−1. This was high enough to explain the oxidation rates calculated from the flux measurements. NH4+ was added to the water overlaying the sediment cores to measure its influence on CH4 oxidation. This treatment resulted in increased concentrations of pore water NH4+ which were measured with a NH4+ microelectrode. Pore water NH4+ concentrations of 20 mM. Measurements of potential NH4+ oxidation rates were below the detection limit of 0.5 nmol NO2− produced mlsed−1 h−1. Thus, the NH4+ oxidizing activity was about 3 magnitudes lower than the CH4 oxidizing activity, and it is unlikely that NH4+ oxidizers are important for CH4 oxidation in this littoral sediment.